This invention relates to apparatus for comparing voltages with lower and upper thresholds that can be set in first and second voltage divider circuits.
A common technique for comparing voltages with a reference voltage uses differential circuits by which one can determine if a voltage exceeds or falls below a predetermined reference voltage level. The mode of operation for such circuits generally comprises weighting the difference between the input voltage being compared and the fixed reference voltage. If the difference runs through zero, a transistor stage is switched. In this case, a specified output signal is provided over an output stage.
Frequently, a voltage must be compared not with one but with two reference values. This problem arises if, by way of example, an input voltage is to be monitored with respect to dropping below a lower or exceeding an upper threshold. Assuming that as a result of the connection of a differential circuit to the dividing point of a voltage divider circuit, to which the input voltage being compared is applied, the various thresholds can be set through appropriate adjustments of the resistance values, the above problem can be solved by providing two differential circuits, each connected to the dividing point of a first and second voltage divider circuit. The output signals of the two differential circuits then can be logically interconnected in such a manner that a specified output signal is continually available, if the input voltage lies within the range defined by the lower and upper thresholds. However, due to the doubling and linking of the output signals, such an arrangement results in a relatively vast amount of circuitry.
An object of the invention, therefore, is to provide a circuit arrangement for comparing a voltage with an upper and a lower threshold, by which the expense for circuitry required heretofore is reduced.